Automated Design of Stiffened Panels Against Crack Growth and Fracture Among Other Design Constraints

A Fortran IV computer program has been developed to design integrally stiffened two-panel boxes subject to fatigue-crack growth and fracture as well as side constraints, displacements, yielding, and local and general buckling. For crack growth and fracture, stress-intensity factors are compounded from available solutions, and final crack sizes are estimated using R-curve methods for the fracture constraints. An average and integrate strategy is employed for life prediction. Mathematical programming is used for design optimization, and approximation concepts reduce the constraint calculation effort. In the design procedure, multilayered constraint deletion and replacement of original constraint analyses by approximate substitutes define a sequence of approximate subproblems for design solution. For each subproblem, an extended Fiacco-McCormick penalty function is used with a modified Davidon-Fletcher-Powell deflected gradient algorithm for the unconstrained minimizations. The effective slope of crack growth with respect to stress-intensity factor is central to the success of procedures developed. This slope provides a normalizing exponent for the design constraints and a weighting exponent for efficient interpolation and quadrature to predict crack-growth lives. Probabilistic treatment of identified uncertainties in the crack growth ahd fracture analysis, in place of across the board safety factors, has improved design efficiency.